1. Field of the Invention
The present invention relates to a lock-up control system for a lock-up type automatic transmission.
2. Description of the Prior Art
Automatic transmissions are provided with torque converters in their power transmission systems to increase torque from the engine. The torque converter has a pump impeller driven by the engine to rotate an operating oil within the torque converter wherein the rotation of the operating oil causes a turbine runner to rotate under the reaction of a stator thereby multiplying the torque (torque converter state). The torque converter, while in use, cannot help avoiding a slip between its pump impeller and its turbine runner, and therefore the automatic transmission provided with such torque converter in its power transmission system has an advantage of easy operation, but has a drawback of poor power transmission effeciency that leads to poor fuel economy. To alleviate this drawback, there has been proposed a so-called torque converter with a direct clutch (often referred also to as a lock-up torque converter) wherein during a relatively high vehicle speed operation range when the torque variation of the engine does not create a problem the turbine runner is directly connected to the pump impeller (lock-up state) thereby eliminating slip therebetween. A lock-up type automatic transmission provided with a torque converter of this kind in a power transmission system thereof is already employed in some vehicles.
In the case of an automatic transmission having a torque converter which locks up whenever the vehicle operates in one of the forward gear ratios and at a vehicle speed exceeding a predetermined vehicle speed which is set for operation in such gear ratio, since the lock-up ranges adjoin one another or overlap one another with respect to vehicle speed, when the vehicle operates with an accelerator pedal depressed deeply, gear shifting takes place with the torque converter in the lock-up state when the automotive vehicle operates within any one of the lock-up ranges and with the accelerator pedal depressed deeply. If the gear shifting takes place with the torque converter left in the lock-up state, the torque variation owing to gear shifting is not absorbed by the torque converter, thus allowing a great shock to take place upon gear shifting.
Accordingly, it has been proposed to temporally release the lock-up state during gear shifting operation even when the vehicle is operating within one of the lock-up ranges so as to allow the torque converter to operate in the torque converter state. For this purpose, a gear shifting detector is provided that generates a gear shift signal having a predetermined time duration in response to and after the occurrence of a command for gear shifting between adjacent two gear ratios, thereby temporarlly releasing the lock-up state while the shift signal is present.
In the proposed system, however, the duration of generation of a gear shift signal from each ear shifting detector circuit is constant, so that the suspention of the lock-up state which is caused by the gear shift signal continues during a constant duration irrespective of kinds of gear shifting commands. The time duration for the automatic transmission to carry out the acutal gear shifting operation differs, owing to the construction of a hydraulic circuit, depending upon the kind of gear shifting commands, viz., which friction unit or units are involved in the particular shifting in the automatic transmission, or depending upon an upshift or downshift even with the same adjacent two gear ratios. Therefore, if the lock-up suspending time duration is constant, even though it might conform to the proper time duration for a certain gear shifting operation, it does not conform to the proper time duration for other gear shifting operations, and is longer or shorter than the actual gear shifting duration, thus causing the occurrence of a gear shift shock or driving feel impairment.